This invention relates to controlling the phase of light exiting a liquid crystal device and, in particular, to an adaptive liquid crystal lens that is controlled electrically.
Heretofore, most liquid crystal devices have been used for display purposes, as for example, in digital time pieces and the like. The device usually includes an electro-optical cell similar to that described in U.S. Pat. No. 3,977,767. The cell typically consists of a pair of spaced apart glass plates or windows, a series of transparent control electrodes in a seven electrode configuration that are disposed on the inside of the plates and a suitable liquid crystal material sandwiched between the electrodes that responds to a voltage applied to the electrodes. Usually, the cell is designed so that no image information is displayed when a voltage below the threshold voltage of the liquid crystal is applied to the electrodes. A voltage above the saturation voltage of the material is applied to selected electrodes which in turn changes the index of refraction of the material in the electrodes region to create a desired image pattern. The term threshold voltage, as herein used, refers to some initial electrode voltage at which the liquid crystal molecules start to react to an applied force field and begin to reorientate themselves in the field. Saturation, on the other hand, refers to a higher electrode voltage at which any molecule reorientation is substantially completed and any further increase in voltage produces little or no effect in the material.
It should be noted that the control electrodes in most display devices are arranged to operate at two levels. The first level is somewhere below threshold while the second is maintained between three and five times the threshold voltage. As a consequence, each electrode acts as an on-off switch to produce either an image or no image in the electrode region.
Lotspeich in U.S. Pat. No. 3,424,513 discloses an electro-optical lens wherein incoming light is caused to pass through a Kerr effect substance that is under the influence of at least one quadrapole control unit. The elongated electrodes of the unit are placed parallel to the optical axis of the lens and are electrically interconnected so that a variable control field is established with the Kerr effect material. Because of the quadrapole arrangement, an entering light beam can only be diverged by the lens with the amount of divergence being dependent upon the voltage applied to the electrodes. Normally, the control voltage must be about 20,000 volts in order to produce the desired effects. Because of the high voltages involved and the fact that the device by itself cannot act as a converging lens, the lens is of little or no practical value.
Bricot et al in U.S. Pat. No. 4,037,929 describes the use of a nematic liquid crystal to modify a glass lens. The glass lens consists of a plano element and a convex element between which the liquid crystal is stored. A first transparent solid area electrode is mounted on the inside surface of the flat element and a second similar type electrode is mounted on the inside surface of the convex element. As taught by Bricot et al, the voltage applied to the electrodes is varied between the threshold and saturation voltages of the liquid crystal to change the index of refraction thereof. By changing the index of refraction of the material the focal length of the glass lens can be modified within extremely narrow limits. The lens, however, suffers from all the defects common to glass lenses and because of the two electrode configurations, the lens is restricted to use in conjunction with incoming light that is polarized in one direction only.